The invention concerns a circuit for the amplification, analog signal processing and A/D conversion of signals from a microphone as defined in the preamble to claim 1.
It is known within microphone and audio technology to integrate D/A conversion and microphone amplification in one unit, so that the sampling point is moved as close as possible to the microphone, and herewith reduce signal distortion, noise and hum which can arise with long signal paths. To reduce noise pulses, it is known from patent application GB-A-2 293 740 to build A/D converters and microphone power supplies on the same circuit board, where the microphone power supply works with pulse modulation at a frequency which is derived from the sampling frequency in the A/D converter. This patent application forms the basis for the two-part form of claim 1.
Where a wide range of portable products within telecommunication, video and audiometrics are concerned, as well as hearing aids and other micro-electronics, the weight and the physical dimensions of the equipment play an important role for the equipment""s fields of application and marketability.
The power consumption belongs typically among the important factors which, together with the relevant battery technology, are determinative for precisely the weight and the physical dimensions of the portable equipment. Therefore, in many connections it is decisive that attempts are made to reduce the power consumption as much as possible.
With active microphones, such as electret microphones, these are normally supplied with a constant current which is in the magnitude of 100-600 xcexcA. For the above-mentioned applications, this constitutes a high current consumption. It is therefore a principle object of the present invention to reduce the current consumption.
This is achieved with the invention as defined in claim 1.
According to the invention as defined in claims 2-4, a strongly reduced current consumption is achieved, in that the microphone coupling is provided with current pulses of such a short duration that the microphone current reaches a usable value. The current consumption in such a coupling is typically only 0.01-0.03 xcexcA per duty cycle.
According to the invention as defined in claim 5, a particularly advantageous coupling is achieved, in that the coupling together of the microphone and amplifier in one unit makes a high signal/noise ratio possible.